Production and recovery of acetic acid by plural distillation



Oct- 31, 1967 R c. BINNING ETAL 3,350,445

PRODUCTION-AND RECOVERY OF' ACETIC ACID BY PLURAL DISTILLATION FiledFeb. l2, 1964 HAROLD R.NULL

ATTORNEY C 3H s United States Patent Olltice 3,359,445 Patented Oct. 31,1967 3,350,445 PRODUCTION AND RECOVERY OF ACETIC ACH) BY PLURALDISTLLA'HON Robert C. Binning, St. Louis, Leon E. Rowe, Glendale,

and Harold R. Null, Florissant, Mo., assignors to Monsanto Company, acorporation oi' Delaware Filed Feb. 12, 1964, Ser. No. 344,228

5 Claims. (Cl. 260-533) ABSTRACT OF THE DISCLOSURE This inventionrelates to the production and recovery of acetic acid involving theoxidation of propylene feedstocks with molecular oxygen in a liquidreaction medium consisting of fully esteried polyacyl esters ofpolyhydroxyalkanes, polyhydroxycycloalkanes, polyglycols and mixturesthereof, While maintaining concentrations of polymeric residue in thereaction zone substantially constant at a level above which additionalresidue is inccmpletely oxidized, and recovering said acetic acid bymeans of a recovery train comprising a combination flasher-stripperdistillation let-down system, a polymeric residue removal distillationzone, absorber and various other parallel and serial distillation zonesand recycle streams for separation of acetic acid from other reactionproducts.

The present invention relates to recovery of acetic acid.

In broad aspect the present invention relates to the oxidation ofpropylene with molecular oxygen to produce acetic acid.

One aspect of the present invention involves conducting the saidoxidation in a `liquid phase comprising fully esteritied polyacyl estersof polyols more fully described hereinafter.

Another aspect of this invention concerns a novel recovery system forthe product acetic acid.

Still another aspect of this invention relates `to a noncatalytic oleiinoxidation system to produce and recover acetic acid.

There are numerous methods described in the prior art for producing andrecovering carboxylic acids. Illustrative prior art methods forproducing such acids include various carbonylation procedures, notablythe reaction of olens with carbon monoxide and water. For example, inUS. Patent 2,831,877 an olefin and carbon monoxide are reacted in ananhydrous medium in the presence of a catalyst such as concentratedsulfuric acid or anhydrous hydrogen fluoride or chlorosulfonic acidalone or with boron triuoride. The reaction mixture is then hydrated toproduce .the resultant carboxylic acid. Variations of the above processinclude the use of dilerent catalysts, eg., monohydroxyuoboric acidalone or mixed with phosphoric or sulfuric acids as in U.S. Patent2,876,241 or solid phosphoric acid as described in U.S. Patent3,036,124.

Another patent ('U.S. Patent 2,913,489) describes Vthe reaction ofalcohols and/ or ethers with carbon monoxide to produce carboxylicacids.

Still another prior art processes (US. Patent 2,000,- 878) describes thereaction of propylene with an aqueous alkali metal hydroxide in Water toproduce alkali metal acetates which may be acidied with concentratedH2804 or HCl to recover acetic acid. This process utilizes temperaturesin the range of from 30G-420 C. and basic aluminum compounds ascatalysts, eg., aluminum oxide or hydroxide.

Other prior art methods rely upon the use of acetylene as a startingmaterial to produce acetic acid. Por example, in U.S. Patent 1,128,780,acetylene is reacted with the production and a peroxidizing agent, e.g.,hydrogen peroxide or persulfuric acid in the presence of mercury or amercury corn.- pound. Another patent (US. Patent 1,174,250) describes aprocess for the reaction of acetylene, oxygen and water in the presenceof a mercury compound, an organic acid such as acetic acid and aninorganic acid, eg., phosphoric acid.

Other methods described in the prior art for producing acetic acidinvolve the oxidation of parains with molecular oxygen in a solvent suchas carbon tetrachloride or benzene containing an Oxidation catalyst andinitiator (US. Patent 2,265,948). Another parallln oxidation processdescribes a non-catalytic oxidation, but relies upon a critical ratio ofthroughput rates of oxygen, .paran and a distillate fraction of reactionproducts boiling below 99 C. in the presence of water. (U.S. Patent2,825,740).

Still other methods for producing acetic acid involve the oxidation ofolens With molecular oxygen in the presence of various catalysts. .Forexample, in one process (US. Patent 3,057,915) ethylene is oxidized Withoxygen in the presence of Water vapor and a catalyst cornprising acarrier, a salt of a noble metal, cupric chloride and an oxide of ametal such as iron, manganese and/or cobalt.

Most of the prior art processes heretofore described `suffer one or moredisadvantages in that they require oxidation catalysts, initiators,critical reactant feed rates, expensive or dangerous starting materials,unduly high reaction temperatures or ditcult separation techniques.

It is, therefore, an object of the present invention to provide a liquidphase propylene oxidation process for `the production of and recovery ofacetic acid.

An object of this invention is to provide a non-catalytic directoxidation of propylene with molecular oxygen to produce acetic acid in aliquid phase comprising fully esteried polyacyl esters of polyols.

Still another object of this invention is to provide a process forproducing acetic acid which does not utilize expensive, unsafe orhard-to-obtain starting materials, is simple, economical and practical.

These and other objects of the invention Will become more apparent asthe description of the invention proceeds.

A schematic flow diagram of the process is shown in the accompanyingdrawing.

The present invention comprises the production of acetic acid by thecontrolled direct oxidation of propylene with molecular oxygen in theliquid phase and to a novel means of separating and recovering thisproduct.

The liquid phase in which the oxidation occurs comprises solvents whichare essentially chemically indifferent, high boiling With respect tovolatile oxidation products and are oxidatively and thermally stableunder the condition of the reaction described. Further, the solventsemployed in the present invention are highly resistant to attack by freeradicals which are generated in the oxidation process. Moreover, thesolvents employed in the instant invention are elective in assuaging thedeleterious eiiects of acidic components, especially formic acid and toa lesser degree acetic acid, on commercially valuable non-acidicby-products, e.g., propylene oxide, which are formed in the oxidation ofolens.

Solvents primarily and preferably contemplated herein comprise fullyesteried polyacyl esters of polyhydroxyalkanes, polyhydroxycycloalkanes,polyglycols and mixtures thereof. Polyacyl esters contemplated hereincontain, generally, from 1 to 18 carbon atoms in each acyl moiety andfrom 2 to 18 carbon atoms in each alkylene or cycloalkylene moiety.However, best results obtain when the acyl moiety contains from l to 6carbon atoms and the alkylene and cycloalkylene moiety each containsfrom 2 to 6 carbon atoms. These esters may be readily prepared bymethods known to the art. For example, in U.S. Patent 1,534,752 isdescribed a method whereby glycols are reacted with carboxylic acids toproduce the corresponding glycol ester. Acid anhydrides may be used inplace of the acids.

Representative glycols include straight-chain glycols, such as ethyleneglycol, propylene glycol, butylene glycol, lpentylene glycol, `hexyleneglycol, heptylene glycol, octylene glycol, nonylene glycol, decyleneglycol, dodecylene glycol, pentadecylene glycol and octadecylene glycol.Branched-chain glycols such as the iso, primary, secondary and tertiaryisomers of the above straight chain glycols are likewise suitable, e.g.,isobutylene glycol, primary, secondary, and tertiary amylene glycols,Z-methyl- 2,4-pentanediol, 2ethyl-l,3hexar1ediol, 2,3-dimethyl-2,3-butanediol, 2methyl2,3butanediol and 2,3-dimethyl-2, 3-dodecanediol.Polyalkylene glycols (polyols) include diethylene glycol, dipropyleneglycol, tripropylene glycol, tetrapropylene glycol and dihexyleneglycol.

In addition to straight and branched-chain glycols, alicyclic glycolssuch as l,2cyclopentanediol, 1,2-cyclohexanediol,l-methyl-1,2-cyclohexanediol and the like may be used. Y

Other suitable hydroxy compounds include polyhydroxy lalkanes, such asglycerol, erythritol and lpentaerythritol and the like.

Representative carboxylic acids include fatty acids such as formic acid,acetic acid, propionic acid, butyric acid, valeric acid, caproic acid,caprylic acid, lauric acid, palmitic acid, stearic acid, naphthenicacids, such as cyclopentane carboxylic acid, cyclohexane carboxylicacid, and aromatic acids such as benzoic acid and the like.

Representative polyacyl esters include polyacyl esters of polyhydroxyalkanes, such as triacyl esters of glycerol, e.g., glycerol triacetate;tetraacyl esters of erythritol and pentaerythritol, eg., erythritoltetraacetate and pentaerythritol tetraacetate and the like, and polyacylesters of polyalkylene glycols (polyglycols), such as diethylene glycoldiacetate, dipropylene glycol diacetate, tetraethylene glycol diacetateand the like. These polyacyl ester solvents may be used individually oras mixtures, being compatible with each other. For example, a mixture ofvarying proportions of a diacyl ester of a hydroxyalkane, such aspropylene glycol diacetate, and a polyacyl ester of a polyglycol, suchas dipropylene glycol diacetate, may be used. Or, a mixture of apolyacyl ester of a polyglycol, such as dibutylene glycol dibutyrate,and a polyacyl ester of a polyhydroxy alkane, such as glyceroltrivalerate, or pentaerythritol tetrapropionate may be used as thesolvent in the instant process illustrated in the examples below. y Ofparticular interest in the present process are the vicinal diacyl estersof alkylene glycols, such as the diformates, diacetates, dipropionates,dibutyrates, divalerates, dicaproates, dicaprylates, dilaurates,dipalmitates and distearates, and mixtures thereof, of the alkylene andpolyalkylene glycols recited above. Still more particularly, of greaterinterest are the diacetates of ethylene and propylene glycols usedindividually or in admixtures of any proportion.

Polyacyl esters having mixed acyl groups are likewise suitable, e.g.,ethylene glycol formate butyrate, propylene glycol acetate propionate,propylene glycol butyrate propionate, butylene glycol acetate caproate,diethylene glycol acetate butyrate, dipropylene glycol propionatecaproate, tetraethylene glycol butyrate caprylate, erythritol diacetatedipropionate, pentaerythritol dibutyrate divalerate, glyceroldipropionate butyrate and cyclohexane- 'cliol acetate v'alerate.

Monoacyl esters of polyhydroxyalkanes and polyglycols are unsuitable foruse as a reaction medium according to the present process. The same istrue of other hydroxy or hydroxylated -compounds such -as glycerin,

glycols, polyglycols and hydroxy carboxylic acids. This is due to thepresence of an abundance of reactive hydroxyl groups which aresusceptible to autooxidative attack, hence, introduce concomitantoxidation side reactions which compete with the desired direct oxidationof the olefin.

ln the preferred mode of operation the polyacyl esters used hereinconstitute the major proportion of the liquid reaction medium withrespect to all other constituents including reactants, oxidationproducts and by-products dissolved therein. By major is meant thatenough solvent is always prese-nt to exceed the combined weight of allother constituents. However, it is within the purview of this invention,although a less preferred embodiment, to operate in such manner that thecombined weight of all components in the liquid phase other thanpolyacyl esters exceeds that of the polyacyl ester solvent. For example,a refinery grade hydrocarbon feedstock or a crude hydrocarbon feedstockcontaining, e.g., 50% by weight of the olefin to be oxidized, e.g.,propylene, and 50% by weight of saturated hydrocarbons, e.g., an alkanesuch as propane, may be used in quantities up to 50% by Weight based onthe solvent. Upon oxidizing this feedstock, unreacted olelin, alkane andoxygen together with oxidation products including acetic acid, lowboilers such as acetaldehyde, propylene oxide, acetone and methylacetate, and high boilers (components having boiling points higher thanthat of the polyacyl ester solvent) formed in the reaction and/orrecycled to the reactor may constitute as much as 75% by weight of theliquid reaction medium, according to reaction conditions or recycleconditions.

When carrying out the invention according to the less preferred mode ofoperation, the quantity of polyacyl ester solvent present in the liquidreaction medium should be not less than 25% by Weight of said medium inorder to advantageously utilize the aforementioned benefitscharacteristic to these unique olefin oxidation solvents.

In further embodiments of the present invention for producing aceticacid by oxidizing olefins `with molecular oxygen in the liquid phase,the polyacyl ester solvents are suitably used in combination withdiluents or `auxiliary solvents which are high boiling with respect tovolatile oxidation products, are yrelatively chemically indifferent andoxidatively and thermally stable under reaction conditions. Here, too,the polyacyl ester solvents should be utilized in quantities not lessthan 25% by weight of the liquid reaction medium in order to retain thesuperior benefits of these polyacyl ester solvents in the present liquidphase olefin oxidation.

Suitable diluents which may be utilized with the polyacyl ester solventsof this invention include, e.g., hydrocarbon solvents such as xylenes,kerosene, biphenyl and the like; halogenated benzenes such aschlorobenzenes, eg., chlorobenzene and the like; dicarboxylic acidesters such as dialkyl phthalates, oxalates, malonates, succinates,adipates, sebacates, eg., dibutyl phthalate, dimethyl succinate,dimethyl adipate, dimethyl sebacate, dimethyl oxalate, dimethyl malonateand the like; aromatic ethers such as diaryl ethers, e.g., diphenylether; halogenated aryl ethers such as 4,4dichlorodiphenyl ether and thelike; diaryl sulfoxides, e.g., diphenyl sulvfoxide; dialkyl and diarylsulfones, e.g., dimethyl sulfone and dixylyl sulfone and nitroalkanes,eg., nitrohexane. While the foregoing have been cited as typicaldiluents which may be used in combination with the polyacyl estersolvents in this invention, it is to be understood that these are notthe only diluents which can be used. ln fact, the benefits accruing fromthe use of these polyacyl esters can be utilized advantageously whensubstantially any relatively chemically indifferent diluent is combinedtherewith.

Therefore, the present'invention in its broadest use comprehends theoxidation of olefin-containing feedstocks in a liquid reaction mediumconsisting essentially of at least by weight based on said medium of atleast one lfully esteried polyacyl ester described above.

In any case, the liquid reaction medium referred to herein is dened asthat portion of the total reactor content which is in the liquid phase.

It is therefore apparent that the liquid reaction media contemplatedherein possess not only those characteristics described in prior artsolvents, viz., they are high boiling with respect to volatile oxidationproducts under the conditions of reaction, essentially chemicallyindifferent and oxidatively and thermally stable, but in addition,possess characteristics not described in prior art oxidations, viz.,resistance to free radical attack, the ability to reduce and/oreliminate the deleterious effects of acidic components on valuablenon-acidic byproducts. In addition, due to the facile manner in whichthe present oxidation proceeds in the described solvents, no oxidationcatalysts, promoters, initiators, buffers, neutralizers, polymerizationinhibitors, etc., are required as in many prior art processes.

As noted above, no added catalysts are required in the present oxidationprocess. However, due to the versatility of the above-described solventsin the olefin oxidations, the usual oxidation catalysts can be toleratedalthough usually no significant benefit accrues from their use. Forexample, metalliferous catalysts such as platinum, selenium, vanadium,iron, nickel, cobalt, cerium, chromium, manganese, silver, cadmium,mercury and their compounds, preferably in the oxide form, etc., may bepresent in gross form, supported or unsupported, or as nely dividedsuspensions.

In like manner, since the olen oxidations according to this inventionproceed at a rapid rate after a brief induction period, no initiators orpromoters are required, but may be used to shorten or eliminate thebrief induction period, after which no additional initiator or promoterneed be added.

Suitable initiators include organic peroxides, such as benzoyl peroxide;inorganic peroxides, such as hydrogen and sodium peroxides; peracids,such as peracetic and perbenzoic acids; ketones, such as acetone;ethers, such as diethyl ether; and aldehydes, such as acetaldehyde,propionaldehyde and isobutyraldehyde.

Use of the solvents described herein, being free of the necessity to usevarious additives described in prior art processes, enhances theseparation and recovery of acetic acid by the sequence of stepsdescribed in detail below.

In carrying out the process of the instant invention, the reactionmixture may be made up in a variety of Ways. For example, the olen andoxygen may be premixed with 4the solvent and introduced into thereactor, or the olen may be premixed with the solvent (suitably, up to50% by weight based on the solvent and, preferably, from 5 to 30% byweight based on the solvent). Preferably, the olen is premixed With -thesolvent and the oxygen-containing gas introduced into the olefin-solventmixture incrementally, or continuously, or the olefin andoxygen-containing gas may be introduced simultaneously through separateor common feedlines into a body of the solvent in a suitable reactionvessel (described below). In one embodiment an olefin andoxygen-containing gas mixture is introduced into the solvent in acontinuously stirred tank reactor, under the conditions of temperatureand pressure described below. Suitable olcnzoxygen volumetric ratios arewithin the range of 1:5 to 15:1. Feed rates, generally, may vary from0.5 to 1500 ft.3/hr., or higher, and will largely depend upon reactorsize. The oxygen input is adjusted in such manner as to prevent anexcess of oxygen 1%) in the olf-gas or above the reaction mixture.Otherwise, a hazardous concentration of explosive gases is present.Also, if the oxygen (or air) feed rate is too high the olefin will bestripped from sidera-tions of the mixture, thus reducing theconcentration of olefin in the liquid phase and reducing the rate ofoxidation of the olefin, hence giving lower conversions per unit time.

Intimate contact of the reactants, olefin and molecular oxygen, in thesolvent is obtained by various means known to the art, e.g., bystirring, shaking, vibration, spraying, sparging or other vigorousagitation of the reaction mixture.

The olen feed stocks contemplated herein include pure propylene,mixtures of propylene with other olelins, c g., ethylene, or olefinstocks containing as much as 50% or more of saturated compounds, e.g.,propane. Olefinic feed materials include those formed by crackinghydrocarbon oils, paran wax or other petroleum fractions such aslubricating oil stocks, gas oils, kerosenes, naphthas and ythe like.

The reaction temperatures and pressures are subject only to those limitsoutside which substantial decomposition, polymerization and excessiveside reactions occur in liquid phase oxidations of propylene withmolecular oxygen. Generally, temperatures of the order of C. to 250 C.are contemplated. Temperature levels sufficiently high to` preventsubstantial build-up of any hazardous peroxides which form are importantfrom consafe operation. Preferred temperatures are Within the range offrom 180 C. to 230 C. Still more preferred operating temperatures arewithin the range of from C. to 210 C. Suitable pressures herein areWithin the range of from 0.5 to 350 atmospheres, i.e., subatmospheric,atmospheric or superatmospheric pressures. However, the oxidationreaction is facilitated by use of higher temperatures and pressures,hence, the preferred pressure range is from 5 to 200 atmospheres. Stillmore preferred pressures are Within the range of from 25 to 75atmospheres. Pressures and ltemperatures selected will, of course, besuch Vas to maintain a liquid phase.

The oxidation of olens, eg., propylene, in the present process isauto-catalytic, proceeding very rapidly after a brief induction period.A typical oxidation of propylene 4requires from abou-t 0.1 to 20minutes.

The reaction vessel may consist of ya wide variety of materials. Forexample, aluminum, silver, nickel, almost any kind of ceramic material,porcelain, glass, silica and various stainless steels, e.g. Hastelloy C,are suitable. It should be noted that in the instant process where noadded catalysts are necessary, no reliance is made upon the walls of thereactor to furnish catalytic activity. Hence, no regard is given toreactor geometry to furnish large-surface catalytic activity.

The oxidation products are removed from the reactor, preferably, as acombined liquid and ygaseous mixture, or the liquid reaction mixturecontaining the oxidation products is removed to -a products separationsystem, a feature of which comprises in combination a asherstripperlet-down arrangement. This arrangement in combination with the precedingpropylene oxidation reaction and With succeeding product-separationsteps constitutes a unique, safe, simple, economic and practical processfor the commercial production and recovery of acetic acid.

In regard to the flasher-stripper let-down system, principal advantagesaccruing from its use are that the system simultaneously (1) utilizesthe heat of the oxidation reaction in the initial separation of gaseousand liquid products; this eliminates the need of cooling the reactoreiuent, (2) minimizes the amount of total overhead solvent, resulting inva reduced solvent load on subsequent distillation columns. Theadvantages of ythis reduced solvent load are that smaller columns arerequired for the requisite products separations; (3) reduces to traceamounts the quantity of acidic components in solvent recycle streams,and (4) removes the bulk of the fixed 75 gases and very volatilecomponents, thus reducing the pressure requirements to prevent excessive-uct in subsequent processing steps.

A particular feature of the asher-stripper let-down combination is thatin the flasher an initial separation of about one-third of the acidsformed in the reaction is accomplished and these are taken overhead; andby use of a stripping column for treatment of the ilasher bottoms,substantially all of the remaining acids, i.e., all but about 0.05 to0.2 wt. percent based on the recycle stream are removed from the recyclesolvent. Advantages afforded by such clean separation of acid values,particularly highly corrosive formic acid, from the recycle solvent arethat all equipment for processing the stripper bottoms can now be madeof plain inexpensive carbon steel, replacing very expensive corrosionresistant stainless steels such as Hastelloy C, and the like, hithertorequired. The economic advantages lare manifest.

The -total effect of the foregoing advantages is to provide aneflicient, rapid economical method for stabilizing the propyleneoxidation reaction mixtures while unloading solvent from the oxidationproducts and recycling solvent to the reactor.

In contrast to the flasher-stripper combination used herein the use ofindividual flashers or distillation columns in the initial separation ofthe products from the reactor eflluent is inadequate for variousreasons. For example, a single ilasher cannot simultaneously minimizethe quan- -tity of overhead solvent, hence reducing the liquid load inthe distillation columns in the separation train, while minimizing theamount of acids in the bottoms stream recycled to the main reactor. Ifconditions of temperature and pressure in a single ilasher are soadjusted :as to permit the desired amount of solvent to go overhead, alarge amount of acids (l wt. percent or more) appear in the bottomsstream and are recycled to the reactor.

Further, when a single distillation column is used in the initialgas-liquid separation of reactor effluent this column must beapproximately five times as large in cross sectional area as that columnused herein into which the combined overhead streams of the flasher andstripper are fed. In feeding the gas-liquid eluent directly into adistillation column a large -amount of fixed gases are present, thusreducing plate efficiency and requiring additional plates whichmaterially adds to the cost of operation. A further disadvantage ofvhaving large quantities of fixed gases in -a distillation columnadjacent to the reactor is that much higher pressures and refrigerants(as opposed to cooling Water) are required to condense overhead gases.

On the other hand, use of a plurality of distillation or strippingcolumns to effect an initial gas-liquid separation of the reactoreffluent is disadvantageous primarily because of the required increasein product hold-time in these columns. This increased hold-timenecessitates longer ex` posure of distillation equipment to thedeleterious action of ormic acid and permits undesired secondaryreactions of by-products as by hydrolysis, esteriiication,polymerization or decomposition. In addition, when no flashers are usedthe total reactor efiluent is loaded into these distillation columnsthus requiring equipment of increased capacity and separationefficiency. Elimination of a asher, moreover, increases capital outlaysince distillation columns are much more expensive than flashers.

The ilasher-stripper let-down combination used herein is in like mannersuperior to let-down arrangements comprising 'a plurality of flashersfor a number of reasons. Primarily, by use of a flasher-strippercombination greater control and flexibility of process operation isassured, it being much easier to change product separationspeciiications and operation 4in a stripper than in a flasher. This is'accomplished principally by controlling the heat input to the stripperfrom a -reboiler. Since a flasher has only one equilibrium stage, astripper magnities by several stages, depending upon the number andeiciency of plates therein, the degree of separation of products loss ofprod- 8 achieved by ashers. A further advantage of using a stripper inplace of a second flasher is that the former removes all but a smallamount, i.e., approximately 0.05 to 0.2 wt. percent, based on totalrecycle stream, of formed acids from the recycle solvent, whereas by useof liashers about 1/2 wt. percent of acids remain in the Irecyclesolvent.

Bottoms from the stripper containing the bulk of the solvent andresidue, i.e., components having boiling points above that of thesolvent, are fed to the top of an absorber to ow downwardcountercurrently to a stream of uncondensed materials from the flasherand stripper overhead which is fed to a lower region of the absorber.

An important feature of the present invention is the controlledoxidation of residue material formed in the main oxidation reactor toincrease the yield of acetic acid produced in the process.

The liquid phase oxidation of hydrocarbons results in the Iproduction ofa complex mixture of oxygenated products. For example, in the presentliquid phase oxidation of propylene with molecular oxygen, over fortyindividual compounds have been identified. In addition to theseindividual compounds, a residue of polymeric material is also produced.This polymeric material is of complex composition and has not been fullycharacterized, but is known to contain a variety of functional groupsincluding carboxyl, carbonyl, alkoxy and hydroxy groups.

When this residue is recycled to the main oxidation reactor it isoxidized primarily to acetic acid rather than carbon oxides and water byproper selection of reaction conditions. However, this residue materialcan be recycled -to and utilized in the reaction zone to produce aceticacid only within certain concentration limits, of which the upper limitis critical. The exceeding of this upper limit is delineated by theoccurrence of a series of interrelated eiects, the rst of which is aviscosity effect. Due to the excessive viscosity of the reactor efuentthe stripper in the product separation train begins to plug up and soonfloods, resulting in a continually decreasing ow of the solvent recyclestream to the oxidation reactor. Concurrent with a reduced solventrecycle ilow, the acetic acid yield drops because decreasing amounts ofresidue, oxidizable to acetic acid, are being returned to the reactor inthe recycle solvent. Also, since less recycle solvent and residue arereturned to the reactor the concentration of unreacted propyleneincreases because of decreasing dilution. This, in turn, results in theoxidation of propylene to still more residue to produce still higherviscosities which further interferes with the normal oxygen distributionand prevents the complete oxidation of residue to acetic acid.

As a result of the foregoing series of events when excess residue ispresent in the reactor, the process soon becomes inoperable.

Therefore, the concentration of residue in the reactor should bemaintained at less than 60%, and preferably, from 25% to 50% by weightbased on the reaction mixture. The process is operable usingconcentrations of residue up to about 75%; however, at residue levelsabove 60% the viscosity of the reaction mixture presents increasingdemands on, and decreasing ability of, auxiliary separation equipment tohandle the load.

According to the present invention, therefore, the concentration ofresidue in the reactor is controlled by use of a residue removal columnoperating on a side stream of the solvent bottoms from the stripper.This control of residue levels in the reactor is achieved by adjustingthe composition of the recycle solvent with respect to the amount ofresidue therein and, hence, in the reactor.

At steady state the solvent recycle stream is monitored to determineresidue level. If the residue level is too high, resulting in theproblems described above, this level can be reduced by increasing theamount of recycle solvent fed to the residue removal column to purge, asbottoms, the excess residue, while solvent is taken overhead, combinedwith the stripper bottoms and fed to the absorber. Conversely, if uponmonitoring the solvent recycle stream it is `t'ound that the residuelevel is too low for maximum acetic acid production, the residue levelin the reactor can be increased by reducing the amount of residue purgedfrom the recycle solvent. Thus, by means of this residue removal columnthe residue content in the recycle solvent can be regulated and, hence,the oxidation in the main oxidation reactor can be controlled to obtaingreater yields of acetic acid.

A further advantage of the use of the residue removal column is that bypurging excess residue from the solvent bottoms from the stripper beforeentering the absorber, the absorbing capacity of the absorber isincreased.

The overhead streams from the liasher and stripper are passed tocondensers from which a stream of condensables is passed to a primaryproduct splitter from which an overhead stream containing propylene,propane and some lower boiling components is removed and passed to apropylene and propane removal zone which serves to separate propyleneand propane overhead from the low boiling oxidation products, eg.,propylene oxide, methyl formate and acetaldehyde which are removed asbottoms. The propylene may be separated from the propane and the formerrecycled to the reactor or, alternatively, the entire overhead from thepropylene-propane removal column processed as described below.

The product acetic acid and other valuable products resulting from thepresent oxidation process are recovered from the bottoms stream of theprimary products splitter referred to above. This bottoms streamcontains all of the solvent taken overhead from the liasher-stripperlet-down system, acid values, water, low boiling components not removedin the primary products splitter overhead stream, including methanol,methyl acetate, acetone, isopropanol, allyl alcohol, biacetyl and othersand some higher boiling components.

The primary products splitter bottoms containing the above values ispassed to a solvent-acid splitter where most of the residual solvent andhigher boiling components are removed as bottoms and recycled to thereactor via the absorber.

Overhead from the solvent-acid splitter containing the remainingresidual solvent, acid values, water and low boiling components ispassed to an acids-low boiler separation column 'where the low boilersand a small amount of water are recovered overhead. These low boilersmay be separated into fractions suitable for various solvent utilities,eg., a methanol, methyl acetate, acetone fraction is useful as a paintthinner or as a film casting solvent. Alternatively, these low boilersmay be separated into individual components such as those mentionedabove by various extraction means such as selective adsorption andfractional desorption, solvent extraction, extractive distillation,azeotropic distillation, etc., using a suitable extractant.

Bottoms from the acids-low boiler separation column containing aceticacid, formic acid, water and residual solvent are passed to anazeotropic distillation column containing benzene. In this columnbenzene forms azeotropes with water and with formc acid which are takenoverhead to a condenser cooled with cooling water. Upon condensing,water and formic acid are cleanly separated from the benzene andcollected in a separator from which benzene is recycled to theazeotropic distillation column, while water and formic acid are removedas bottoms from the separator. Bottoms from the azeotropic distillationcolumn comprising primarily acetic acid and the residual solvent fromthe oxidation reactor are passed to an acetic acid refining column fromwhich purified acetic acid is recovered overhead as a iinal product andthe residual solvent is removed as bottoms and recycled to the absorber,then to the reactor.

A preferred specic embodiment of the present invention will be describedin connection with the direct oxidation of propylene in a continuousoperation and a specic novel method of separating and reiining aceticacid 10 formed in the reaction, reference being made to the accompanyingdrawing. Suitable variations in the separation trains are alsodisclosed.

Example In this process a one-liter Magnedrive autoclave serves as thereactor portion of a continuous system. Solvent, propylene and oxygenare introduced through a bottom port directly below a Dispersimaxturbine agitator operating at 900 r.p.m. The reactor is heatedelectrically and temperature control maintained by modulating Water owthrough internal cooling coils. Reaction temperatures are continuouslyrecorded on a strip-chart.

In operation the reactants, 92% propylene and 99% oxygen, together withpropylene glycol diacetate, a preferred solvent, are fed through line 10to an oxidation reactor 11, operating at 850 p.s.i.g. and 210 C. Themolar feed ratio of C3H6/O2 is 0.8. Total hold time is about S minutes.A variation is to provide two or more reactors in parallel operatingunder identical conditions and feeding the eiiiuent from these reactorsinto the liasher-stripper let-down system described below.

The reaction product, a combined gas-liquid eiuent, is fed continuouslythrough line 12 to a flasher 13 which operated at p.s.i.a. pressure and190 C. at the bottom and C. at the top. From this flasher most of thelow boiling components including all unreacted propylene, CO2 and atleast one-half, and in this example approximately 60%, of the other lowboilers goes overhead along with about one-fourth of the acids, eg.,formic and acetic acids, all dissolved gases and about `6-8% of solvent.Bottoms from the flasher are fed through line 17 to a stripping column18 operating at approximately 24.7 p.s.i.a. and 200 C. at the bottom andusing 6 distillation plates. The residual low boilers, i.e., generallybetween 40% and 50% of that formed, and about 33% in this example,substantially all of the remaining acids, lighter components and 10-15of the solvent are vaporized and taken overhead. Bottoms from thestripper containing the bulk of the solvent are fed through line 19 toan absorber 20. The solvent eiueut from the stripper contains about 50%by weight of residue, i.e., reaction products having boiling pointsabove that of the solvent. A side stream of the solvent eiiuent from thestripper is fed through line 21 to a residue removal stripper column 22by means of which the residue content in the recycle solvent and, hence,the reactor is regulated by purging excess residue as bottoms and thesolvent is removed overhead through line 23 and fed to the absorber viathe stripper bottoms stream, thus increasing the absorbing capacity ofabsorber. The residue removal column is heated to about 153 C. at thetop and 210 C. at the bottom at a pressure of 250 mm. Hg absolute. Sevendistillation plates are used in the residue removal column.

Overhead from the asher and stripper are directed to partial condensers,operating with cooling Water. In the flasher condenser 15uncondensables, including fixed gases, most of the CO2, about 7% of thetotal low boilers, about 74% of the unreacted propylene, and propane areseparated from the condensables and fed through line 16 countercurrentlyto the solvent bottoms 19 from the stripper 18 to the absorber. Theuncondensables from the stripper condenser 14 containing CO2, propaneand propylene are either discarded if desired or, optionally compressedand fed to the absorber to recover the propylene. The absorber isoperated at 150 p.s.i.g. and at temperatures of approximately 75 C. atthe top and 95 C. at the bottom and has twenty-jive plates. Fixed gases,O2, H2, N2, CH4, CO and CO2 are vented from the top of the absorber.Propane, propylene, and other soluble components are absorbed in thesolvent which is recycled to the reactor through line 44 or,alternatively, further processed for propylene purification, as will bediscussed below.

The condensed liquids from the stripper condenser are combined withthose from the asher condenser and this combined stream 25 containing95% of the formed low boilers, most of the acids and about 20% of thesolvent is fed to a primary products splitter, 26, a distillation columncontaining 40 plates and operating at about 40 C. at the top and 210 C.at the bottom under 150 p.s.i.a. pressure and a redux ratio of 0.16. Inthis column lower boiling components including unreacted propylene,propane, acetaldehyde, propylene oxide, methyl formate, and a smallamount of residual CO2 are removed overhead, and water, acids, methanol,acetone, methyl acetate and residual solvent are removed as bottoms. Theprocessing of this bottoms stream to recover acetic acid will bediscussed below.

The overhead stream 27 from the primary products splitter column is fedto a propylene-propane removal column 28. This column is heated to about50 C. at the top and 160 C. at the bottom and maintained at 300 p.s.i.a.and propylene, propane and any residual CO2 are removed overhead whilelow boiling oxidation products such as propylene oxide, acetaldehyde andmethyl formate are removed as bottoms through line 31. Thirty-fourplates and a reflux ratio of 0.31 are used. The overhead from thiscolumn is fed through line 29 to a propane-propylene splitter column 30.This column is heated to approximately 50 C. at the top and 55 C. at thebottom under 300 p.s.i.a. Seventy-live plates and a reflux ratio of 11.7are utilized. Propane is removed from the bottom through line 34 andpropylene is taken overhead through line 3S and recycled to the reactor.Some propane may be driven overhead, if desired, for recycle byincreasing the temperature at the bottom of this column.

An alternative procedure for removing propane from recycle propylene isto combine the overhead from the propylene-propane removal column withthe overhead stream from the condensers leading to the absorber. Asmentioned previously, the liquid bottoms from the absorber containingsolvent, propylene and propane may be recycled directly to the reactoror further processed for propylene purication, i.e., propane removal.When the concentration of propane in the reactor tends to build up to alevel which interferes with the propylene oxidation, additional, orexcess, propane is prevented from being recycled to the reactor bydirecting the effluent bottoms from the absorber, wholly or partially,through a side-stream taken from the absorber bottoms stream, by meansof a distributing valve into a desorber operated at about 50 C. at thetop and 100 C. at the bottom and 300 p.s.i.a. pressure. Here, solvent isremoved as bottoms and recycled to the reactor and propane and propyleneare removed overhead to a C3Hg-C3H3 splitter operating at 300l p.s.i.a.and heated to about 50 C. at the top and 55 C. at the bottom. Propane isremoved as bottoms and propylene of essentially the same composition asthe initial feed material is recycled to the reactor propylene feedstream.

Turning now to the recovery of the acetic acid product and othervaluable oxygenated by-products, reference is made to the bottoms stream50 from the primary products splitter 26 described above. This streamcontains all of the solvent taken overhead from the asher-stripperletdown system, acid values, water, low boiling components not removedin the primary products splitter tops stream, including methanol, methylacetate, acetone, isopropanol, allyl alcohol, biacetyl and others,various high boiling components including acetonyl acetate, and a smallamount of residue. This stream is fed to a solvent-acids splitter 51.

From the solvent-acids splitter, which has plates and operates at about105 C. at the top and 192 C. at the bottom under p.s.i.a. pressure andusing a reflux ratio of 3.0, most of the residual solvent and highboiling components -are removed as bottoms through line 52 and recycledto the reactor via the absorber. The overhead product from thesolvent-acids splitter containing the remaining residual solvent, acidvalues, water and low boiling components is passed through line 53 to an12 acids-low boilers separation distillation column 54 where the lowboiling components and a small amount of water are recovered overheadthrough line 55. This column utilizes 60 plates and operates at about 88C. at the top and 116 C. at the bottom under 15 p.s.i.a. pressure and areflux ratio of 8.0.

Bottoms from the acids-low boilers separation column containingprimarily acetic acid, formic acid, Water and a small amount of residualsolvent are directed through line 56 to an azeotropic distillationcolumn 57 containing about 70 trays and operating at about 77 C. at thetop and C. at the bottom under 15 p.s.i.a. pressure. Benzene is used asan azeotrope-former and is fed through line 61 to the column at a pointabove the top tray at a ratio of 9 parts by weight of benzene for eachpart of overhead product from the column. Uniquely, in this systembenzene forms two distinct azeotropic mixtures; one with water and onewith formic acid, rather than a ternary azeotrope of these threecomponents. In operation, a benzene-water azeotrope and a benzene-formicacid azeot-rope are removed overhead through line 58 to `a condenser 66(circulating water). Upon condensing, a mixture of benzene, Water andformic acid are passed through line 67 to a collector 59 wherein themixture separates into an upper benzene phase and a lower phasecontaining about 42% water, 55% formic acid and about 3% acetic acid.The latter components are removed from the bottom of the collector whilebenzene from the upper phase (replenished with make-up benzene throughline 60) is recycled through line 61 to the azeotropicdistillationcolumn.

Meanwhile, acetic acid is removed as the bulk of the bottoms (over 86weight percent) from this column through line 62 together with smallamounts of residual solvent from the main oxidation reactor to `anacetic acid refining column l63 having 40 trays and operating at about118 C. at the top and 130 C. at the bottom under 15 p.s.i.a. pressureand a reflux ratio of 5.0. Purified acetic acid is recovered overheadthrough line 65 while the residual oxidation solvent is removed as abottoms stream. This stream 64 is combined with the bottoms 52 from thesolvent-acids splitter 51 and from the stripper 19 and fed to theabsorber 20 and then recycled t0 the oxidation reactor by way of theabsorber bottoms 44. This absorber bottoms stream contains about 45% byweight of residue upon entering the reactor. Under the conditions ofoperation in this embodiment the amount of residue purged from therecycle solvent is lcontrolled to maintain a relatively constant residuelevel in the reactor.

In a typical oxidation according to the present embodiment feedmaterials are added to the main oxidation reactor at approximately thefollowing hourly rates: propylene, 575 g., oxygen, 700 g. and solvent(e.g., propylene glycol diacetate), 4,600 g. At steady state (reactorresidence time about 8.0 minutes) propylene conversion is about 50% andoxygen conversion 99.95%. Acetic acid is obtained in about 36 molepercent yield based on propylene reacted, together with minor amounts ofother oxygenated products.

Solvent losses due to mechanical operation of the process are made up byadding fresh solvent to the system as needed, preferably, to the solventrecycle line line entering the absorber.

While the invention has been specifically described with reference tothe oxidation of propylene and recovery of acetic acid and othervaluable oxygenated products, it is within the purview of the inventionto utilize the above-described and illustrated system for the oxidationof other ole'finic compounds to carboxylic acids and recovery thereof,together with associated oxygenated products similarly as describedabove- It being understood that process conditions, e.g., temperaturesand pressures in the reactor, asher, stripper, columns, etc,

will be modified accordingly to make the necessary separations.

Other olens suitable for use herein preferably include those of theethylenic and cycloethylenic series up t 8 carbon atoms per molecule,eg., ethylene, propylene, butenes, pentenes, hexenes, heptenes andoctenes; cyclobutenes, cyclopentenes, cyclohexenes, cyclooctenes, etc.Of particular interest, utility and convenience are acyclic olenscontaining from 2 to 8 carbon atoms. Included lare the alkyl-substitutedolefins such as 2-methyl1 butene, Z-methyl-Z-butene, Z-methyl-propene,4-methyl- 2-pentene, 2,3dimethyl-2-butene and Z-methyl-Z-pentene. Othersuitable olenic compounds include dienes such as butadiene, isoprene,other pentadienes and hexadienes; cyclopentenes, cyclohexenes,cyclopentadiene, vinyl-substituted cycloalkenes and benzenes, styrene,methylstyrene, and other vinyl-substituted aromatic systems.

It is to be understood that the foregoing detailed description is merelyillustrative of the invention and that many variations will occur tothose skilled in the art without departing from the spirit and scope ofthis invention.

What is claimed:

1. Process for the production of acetic acid which comprises oxidizingpropylene feedstocks with molecutar oxygen in a solvent selected fromthe group consisting of fully esteriiied polyacyl esters ofpolyhydroxyalkanes, polyhydroxycycloalkanes, polyglycols and mixturesthereof, where said esters contain from 1 to 18 carbon atoms in eachacyl moiety and from 2 to 18 carbon atoms in each alkylene andcycloalkylene moiety, under temperatures and pressures sulicient tocause the reaction to proceed in the liquid phase and recovering saidacetic acid by:

(a) directing etliuent stream of the reaction mixture from a reactionzone through a combination letdown distillation zone comprising aflashing zone followed by a stripping zone, said ashing zone andstripping zone being maintained at pressures substantially lower than ineach preceding zone and at temperatures necessary to separatesubstantially all of the acetic acid and lower boiling products overheadas gas phase and higher boiling components including the bulk of thesolvent and residue which are removed as bottoms from said strippingzone,

(b) passing said overhead gas phase to condensing zones, from whenceuncondensed gases are directed to an absorbing zone into which thebottoms stream from said stripping zone is also passed to absorbuncondensed propylene, propane and yminor amounts of oxygenatedComponents; removing vent gases overhead from said absorber, whilefeeding the bottoms stream from said absorbing zone back to saidreaction zone as recycle solvent,

(c) directing a side-stream of said bottoms from said stripping zonethrough a residue removal distillation zone wherein the concentration ofresidues of polymeric reaction products having boiling points above thatof said solvent in said recycle solvent is purge regulated to admitsucient residue into said reaction zone as to maintain a residueconcentration therein not exceeding that level above which additionalresidue is incompletely oxidized, and wherein excess residue is removedas bottoms and solvent 1s distilled overhead and recombined with thesolvent bottoms from said stripping Zone and fed to said absorbing zone,

(d) feeding a combined stream of condensed liquids from said condensingzones into a primary products distillation splitting zone from which anoverhead stream containing unreacted propylene, propane and some lowboilers is removed to a propylene-propane removal zone from which saidlow boilers are removed as bottoms while propylene and propane areremoved overhead and fed to a distillation splitter for these componentsand wherein propane is removed as bottoms and propylene is removedoverhead and recycled to said reaction zone,

(e) feeding the bottoms from said primary products distillationsplitting zone in step (d) to an acidsolvent distillation splitting zonewhere most of the residual solvent from said reaction zone not removedin said flashing and stripping zones and higher boiling components areremoved as bottoms and recycled to said absorber,

(f) feeding the overhead from said acid-solvent distillation splittingzone containing residual solvent, acid values, water and low boilers toan acids-low boilers distillation separation zone wherein the lowboilers and some water are recovered overhead, while directing thebottoms from said acids-low boilers separation zone to an azeotropicdistillation column using benzene as an azeotrope-former for water andformic acid,

(g) removing from said azeotropic distillation zone an overhead streamcontaining a mixture of benzenewater and benzene-formic acid azeotropesto a condensing zone wherein benzene is separated from water and -formicacid and feeding these three components to a collecting zone in whichbenzene forms an upper phase from which benzene is returned to saidazeotropic distillation zone, while water and formic acid are removed asa lower phase, and

(h) removing from said azeotropic distillation zone a bottoms streamcontaining primarily acetic acid and a small amount of residual solventto a refining zone wherein purified acetic acid is recovered overheadand said residual solvent is removed aS bottoms and recycled to saidabsorber.

2. Process according to claim 1 wherein said solvent comprises a vinicaldiacyl ester of a polyhydroxyalkane.

3. Process according to claim 2 wherein said solvent comprises propyleneglycol diacetate.

4. Process according to claim 1 tion occurs at temperatures Within therange of from C. to 250 C. and pressures within the range of from 0.5atmosphere to 350 atmospheres.

5. Process according to claim 4 wherein said oxidation occurs in theabsence of added catalysts.

wherein said oxida- References Cited UNITED STATES PATENTS 1,813,6367/1931 Petersen et al 203-69 2,224,984 12/1940 Potts et al. 203-882,658,863 11/1953 Guala 203-88 2,744,939 5/1956 Kennel 203-88 2,985,6685/1961 Shingu 260-533 3,024,170 3/1962 Othmer et al. 203-67 3,071,6011/1963 Aries 260-533 3,153,058 10/1964 Sharp et al. 26o-348.5

NORMAN YUDKOFF, Primary Examiner. WILBUR L. BASCOMB, Examiner.

1. PROCESS FOR THE PRODUCTION OF ACETIC ACID WHICH COMPRISES OXIDIZINGPROPYLENE FEEDSTOCKS WITH MOLECULAR OXYGEN IN A SOLVENT SELECTED FROMTHE GROUP CONSISTING OF FULLY ESTERIFIED POLYACYL ESTERS OFPOLYHYDROXYALKANES, POLYHYDROXYCYCLOALKANES, POLYGLYCOLS AND MIXTURESTHEREOF, WHERE SAID ESTERS CONTAIN FROM 1 TO 18 CARBON ATOMS IN EACHACYL MOIETY AND FROM 2 TO 18 CARBON ATOMS IN EACH ALKYLENE ANDCYCLOALKYLENE MOIETY, UNDER TEMPERATURES AND PRESSURES SUFFICIENT TOCAUSE THE REACTION TO PROCEED IN THE LIQUID PHASE AND RECOVERING SAIDACETIC ACID BY: (A) DIRECTING EFFLUENT STREAM OF THE REACTION MIXTUREFROM A REACTION ZONE THROUGH A COMBINATION LETDOWN DISTILLATION ZONECOMPRISING A FLASHING ZONE FOLLOWED BY A STRIPPING ZONE, SAID FLASHINGZONE AND STRIPPING ZONE BEING MAINTAINED AT PRESSURES SUBSTANTIALLYLOWER THAN IN EACH PRECEDING ZONE AND AT TEMPERATURES NECESSARY TOSEPARATE SUBSTANTIALLY ALL OF THE ACETIC ACID AND LOWER BOILING PRODUCTSOVERHEAD AS GAS PHASE AND HIGHER BOILING COMPONENTS INCLUDING THE BULKOF THE SOLVENT AND RESIDUE WHICH ARE REMOVED AS BOTTOMS FROM SAIDSTRIPPING ZONE, (B) PASSING SAID OVERHEAD GAS PHASE TO CONDENSING ZONES,FROM WHENCE UNCONDENSED GASES ARE DIRECTED TO AN ABSORBING ZONE INTOWHICH THE BOTTOMS STREAM FROM SAID STRIPPING ZONE IS ALSO PASSED TOABSORB UNCONDENSED PROPYLENE, PROPANE AND MINOR AMOUNTS OF OXYGENATEDCOMPONENTS; REMOVING VENT GASES OVERHEAD FROM SAID ABSORBER, WHILEFEEDING THE BOTTOMS STREAM FROM SAID ABSORBING ZONE BACK TO SAIDREACTION ZONE AS RECYCLE SOLVENT, (C) DIRECTING A SIDE-STREAM OF SAIDBOTTOMS FROM SAID STRIPPING ZONE THROUGH A RESIDUE REMOVAL DISTILLATIONZONE WHEREIN THE CONCENTRATION OF RESIDUES OF POLYMERIC REACTIONPRODUCTS HAVING BOILING POINTS ABOVE THAT OF SAID SOLVENT IN SAIDRECYCLE SOLVENT IS PURGE REGULATED TO ADMIT SUFFICIENT RESIDUE INTO SAIDREACTION ZONE AS TO MAINTAIN A RESIDUE CONCENTRATION THEREIN NOTEXCEEDING THAT LEVEL ABOVE WHICH ADDITIONAL RESIDUE IS INCOMPLETELYOXIDIZED, AND WHEREIN EXCESS RESIDUE IS REMOVED AS BOTTOMS AND SOLVENTIS DISTILLED OVERHEAD AND RECOMBINED WITH THE SOLVENT BOTTOMS FROM SAIDSTRIPPING ZONE AND FED TO SAID ABSORBING ZONE, (D) FEEDING A COMBINEDSTREAM OF CONDENSED LIQUIDS FROM SAID CONDENSING ZONES INTO A PRIMARYPRODUCTS DISTILLATION SPLITTING ZONE FROM WHICH AN OVERHEAD STREAMCONTAINING UNREACTED PROPYLENE, PROPANE AND SOME LOW BOILERS IS REMOVEDTO A PROPYLENE-PROPANE REMOVAL ZONE FROM WHICH SAID LOW BOILERS AREREMOVED AS BOTTOMS WHILE PROPYLENE AND PROPANE ARE REMOVED OVERHEAD ANDFED TO A DISTILLATION SPLITTER FOR THESE COMPONENTS AND WHEREIN PROPANEIS REMOVED AS BOTTOMS AND PROPYLENE IS REMOVED OVERHEAD AND RECYCLED TOSAID REACTION ZONE, (E) FEEDING THE BOTTOMS FROM SAID PRIMARY PRODUCTSDISTILLATION SPLITTING ZONE IN STEP (D) TO AN ACIDSOLVENT DISTILLATIONSPLITTING ZONE WHERE MOST OF THE RESIDUAL SOLVENT FROM SAID REACTIONZONE NOT REMOVED IN SAID FLASHING AND STRIPPING ZONES AND HIGHER BOILINGCOMPONENTS ARE REMOVED AS BOTTOMS AND RECYCLED TO SAID ABSORBER, (F)FEEDING THE OVERHEAD FROM SAID ACID-SOLVENT DISTILLATION SPLITTING ZONECONTAINING RESIDUAL SOLVENT, ACID VALUES, WATER AND LOW BOILERS TO ANACIDS-LOW BOILERS DISTILLATION SEPARATION ZONE WHEREIN THE LOW BOILERSAND SOME WATER ARE RECOVERED OVERHEAD, WHILE DIRECTING THE BOTTOMS FROMSAID ACIDS-LOW BOILERS SEPARATION ZONE TO AN AZEOTROPIC DISTILLATIONCOLUMN USING BENZENE AS AN AZEOTROPE-FORMER FOR WATER AND FORMIC ACID,(G) REMOVING FROM SAID AZEOTROPIC DISTILLATION ZONE AN OVERHEAD STREAMCONTAINING A MIXTURE OF BENZENEWATER AND BENZENE-FORMIC ACID AZEOTROPESTO A CONDENSING ZONE WHEREIN BENZENE IS SEPARATED FROM WATER AND FORMICACID AND FEEDING THESE THREE COMPONENTS TO A COLLECTING ZONE IN WHICHBENZENE FORMS AN UPPER PHASE FROM WHICH BENZENE IS RETURNED TO SAIDAZEOTROPIC DISTILLATION ZONE, WHILE WATER AND FORMIC ACID ARE REMOVED ASA LOWER PHASE, AND (H) REMOVING FROM SAID AZEOTROPIC DISTILLATION ZONE ABOTTOMS STREAM CONTAINING PRIMARILY ACETIC ACID AND A SMALL AMOUNT OFRESIDUAL SOLVENT TO A REFINING ZONE WHEREIN PURIFIED ACETIC ACID ISRECOVERED OVERHEAD AND SAID RESIDUAL SOLVENT I REMOVED AS BOTTOMS ANDRECYCLED TO SAID ABSORBER.